Yes, I’ve revisited this blog topic and indeed the rule I follow is: mixer strain sound means speed limit.
I was not aware of the Speed 2 limit on the KA stand mixers until recently when I did my 40 batch panettone at speeds up to 8! I have the Professional 600 with the spiral dough hook, the issue with this model is not the motor (the motor has plenty power), the issues are the gears and gear cover that will wear off or crack.
In addition, the spiral dough hook causes an up/down motion, desirable for kneading dough (the closest a machine can imitate kneading by hand), but undesirable for the gear cover box which cracked up open at Speed 8!
Whenever a dough recipe calls for higher than Speed 2 on a stiff dough, or whenever I hear the mixer strain sound, I use the mixer only for the initial kneading, then stop and continue/help kneading by hand. You will be surprised: kneading by hand is not that difficult after the mixer has done the initial kneading.